1. Field of the Invention
The present invention relates to a transmission line propagation delay time measuring device for measuring the propagation delay time in transmission lines, more specifically to a technique for correcting the propagation delay time measured by TDR (Time Domain Reflectometer) methods.
2. Conventional Art
Next, a transmission line propagation delay time measuring device according to the conventional art shall be explained using FIGS. 6-8.
FIG. 6 is a block diagram showing an example of the structure of a conventional transmission line propagation delay time measuring device. FIG. 7 is a flow chart showing the procedural flow of the measuring method of FIG. 6. FIG. 8 is a waveform diagram showing an example of this procedure.
In FIG. 6, reference numeral 1 denotes a test signal generating circuit, reference numeral 2 denotes a measurement point at which waveform detection is performed, reference numeral 3 denotes a transmission line, reference numeral 4 denotes an open end of the transmission line 3, reference numeral 5 denotes a timing measuring circuit for measuring the time required for the waveform to reach an arbitrary voltage value, and reference numeral 8 denotes a computing circuit for performing the computation procedures of the propagation delay time based on the measured time.
Additionally, test signal 10 in FIG. 8 is the waveform of the test signal used for measurement, which is outputted by the test signal generating circuit 1. The voltage amplitude of the test signal 10 is 0-V.sub.h, and the rise time is T.sub.r. Additionally, waveform 11 is the waveform at the open end 4, and waveform 12 is the waveform at the measurement point 2.
Next, the operations of the conventional art shall be explained using FIGS. 6-8.
First, a test signal 10 is supplied from the test signal generating circuit 1 to the transmission line 3.
Then, in order to determine the time at which the test signal 10 was supplied to the transmission line 3, the timing measuring circuit 5 measures the time T.sub.1 at which the voltage of the waveform 12 at the measurement point 2 reaches voltage V.sub.2 (S200).
Next, in order to determine the time at which the reflected wave reflected by the open end 4 returns, the timing measuring circuit 5 measures the time T.sub.2 at which the voltage of the waveform 12 at the measurement point 2 reaches a voltage V.sub.3 (S201).
Then, the computing circuit 8 calculates the difference T.sub.2 -T.sub.1 between time T.sub.1 and time T.sub.2, as the propagation delay time T.sub.rf1 for making a round trip of the transmission line 3.
Finally, the computing circuit 8 divides the value of the propagation delay time T.sub.rf1 for making a round trip by 2, to calculate the propagation delay time T.sub.pd to the open end 4 of the transmission line 3.
However, this conventional art works only under the assumption of an ideal case in which there is no attenuation on the transmission line, attenuation of the signal does occur on the transmission line. For this reason, there are large measurement errors in the measurement method of the conventional art, and it is not possible to make any measurements of propagation delay time with high precision.
Next, in order to clarify this point, the case wherein attenuation occurs on the transmission line will be explained with reference to FIG. 9.
FIG. 9 is a waveform diagram showing an example of the procedure for the case wherein attenuation occurs on the transmission line 3.
In this drawing, waveform 13 is the waveform at the open end 4 when attenuation occurs in the transmission line 3 and the waveform is blunted, and waveform 14 is the waveform at the measurement point 2 when attenuation occurs in the transmission line and the waveform is blunted. Everything else is the same as FIG. 8.
In this case, as shown in FIG. 9, when the waveform is blunted as with waveform 13 at the open end and waveform 14 at the measurement point, the propagation delay time T.sub.rf1 for the round trip is not equal to twice the propagation delay time T.sub.pd to the open end 4, and a measurement error T.sub.error occurs. For example, in the case of short time measurements on the order of nanoseconds, measurement errors T.sub.error of several hundred picoseconds can be expected.